Wednesday, May 15, 2013

A
satellite image of Maupiti, one of the Society Islands, which is on its
way to becoming an atoll. Submerged reef appears in pale blue.>>> geolocalization with the Marine GeoGarage <<<CREDIT: NASA Earth Observatory

Charles Darwin sparked more than one controversy over the natural
progression of life.
One such case involved the evolution of coral
atolls, the ring-shaped coral reefs that surround submerged tropical
islands.

Coral reefs are actually huge colonies of tiny animals that need
sunlight to grow.
After seeing a reef encircling Moorea, near Tahiti,
Darwin came up with his theory that coral atolls
grow as reefs stretch toward sunlight while ocean islands slowly sink
beneath the sea surface. (Cooling ocean crust, combined with the weight
of massive islands, causes the islands to sink.)

A century-long controversy ensued after Darwin published his theory in
1842, because some scientists thought the atolls were simply a thin
veneer of coral, not many thousands of feet thick as Darwin proposed.
Deep drilling on reefs finally confirmed Darwin's model in 1953.

But reef-building is more complex than Darwin
thought, according to a new study published May 9 in the journal
Geology.
Although subsidence does play a role, a computer model found
seesawing sea levels, which rise and fall with glacial cycles, are the
primary driving force behind the striking patterns seen at islands
today.
"Darwin actually got it mostly right, which is pretty amazing," said
Taylor Perron, the study’s co-author and a geologist at MIT. However,
there’s one part Darwin missed. "He didn't know about these glacially
induced sea-level cycles," Perron told OurAmazingPlanet.

What happens when sea-level shifts get thrown into the mix?
Consider Hawaii
as an example.
Coral grows slowly there, because the ocean is colder
than in the tropics.
When sea level is at its lowest, the Big Island
builds up a nice little reef terrace, like a fringe of hair on a balding
pate.
But the volcano — one of the tallest mountains in the world, if
measured from the seafloor — is also quickly sinking.
Add the speedy
sea-level rise when glaciers melt, and Hawaii's corals just can't keep
up.
The reefs drown each time sea level rises.

The computer model accounts for the wide array of coral reefs seen at islands around the world — a variety Darwin's model can't explain, the researchers said.
"You can explain a lot of the variety you see just by combining these
various processes — the sinking of islands, the growth of reefs, and the
last few million years of sea level going up and down rather
dramatically," Perron told OurAmazingPlanet.
For nearly 4 million years, Earth has cycled through global chills,
when big glaciers suck up water from the oceans, and swings to
sweltering temperatures that melt the ice, quickly raising sea level.
This cyclic growth of ice sheets takes about 100,000 years.
The researchers also found that one of the few places in the world
where sinking islands and sea-level rise create perfect atolls is the
Society Islands, where Darwin made his historic observations.